1. Field of the Invention
The present invention relates to resonant frequency measurement, in general, and more particularly, to a method of determining a resonant frequency of a mechanical device including a first mass and at least one second mass mechanically coupled thereto.
2. Description of the Prior Art
It is often desirable to determine the resonant modes of a quartz crystal arrangement where the frequency information may be used in scientific study, for example. Frequency characteristics of interest may include resonant frequencies, inharmonic overtones, and harmonic modes of the crystal arrangement.
Historically, methods of measuring resonant modes of a crystal include swept frequency sinusoidal analysis and crystal oscillator testing. The swept frequency sinusoidal analysis involves applying a sinusoidal signal to the crystal arrangement and sweeping the frequency of the sinusoidal input signal through a frequency range of interest. Based on the response of the crystal arrangement to the sinusoidal input, certain frequency characteristics can be obtained. The swept frequency sinusoidal analysis, however, is often tedious and overly time consuming, and often requires repetitive cycles of: providing an input signal to the crystal arrangement, changing the frequency of the input signal, measurement of the response of the crystal arrangement, comparison of the gathered information, and determining the input signal to be applied to the crystal arrangement for a subsequent measurement based on the current cycle measurement. Typically, for a high accuracy measurement very precise and expensive measurement equipment must be used. However, even when measurements are taken with the very precise and expensive equipment, errors may still arise due to human intervention during the analysis.
The crystal oscillator testing method for measuring a resonant mode of a crystal involves placing the crystal in an electronic oscillator circuit and measuring the oscillator signal frequency. Due to the resonant properties of a crystal, the electronic oscillator circuit will converge to the crystal""s resonant frequency. While crystal oscillator testing is a reasonable method to determine the principal resonant frequency of a crystal, it is an undesirable method where additional resonant modes are to be measured.
It is one object of this invention to provide a method and apparatus to rapidly and accurately determine the frequency of a desired resonant mode characteristic of a crystal arrangement, or other two port device, under test without the use of complicated and expensive test equipment.
It is another object of this invention to provide a method and apparatus to rapidly determine the frequency of a desired resonant mode of a crystal arrangement, or other two port device, under test as part of a fully automated operation.
It is another object of this invention to provide a method and apparatus to rapidly determine the frequency of a desired resonant mode of a crystal arrangement under test while discriminating between several different modes of oscillation of the crystal.
It is another object of this invention to provide a method and apparatus for measuring multiple, closely spaced resonant frequencies of a crystal arrangement, or other two port device, under test.
It is another object of this invention to provide a precision, linear crystal-controlled oscillator utilizing an inexpensive crystal and a voltage controlled oscillator.
In accordance with the present invention, a method for determining a resonant frequency of a mechanical device having a first mass and at least one second mass mechanically coupled to the first mass comprises the steps of: calculating operating parameters related to the resonant frequency of the mechanical device, the operating parameters comprising a frequency range of operation, a final error value, and a threshold value; providing a control signal to a voltage-controlled oscillator, wherein the frequency of an output of the voltage-controlled oscillator is responsive to the control signal, and the frequency of the output of the voltage-controlled oscillator is within the calculated frequency range of operation; phase shifting the output of the voltage-controlled oscillator; translating the phase shifted output of the voltage-controlled oscillator into an oscillatory force; applying the oscillatory force to one of the masses of the mechanical device to cause the mechanical device to respond at a frequency and amplitude; measuring the response of the mechanical device and generating a response signal representative thereof in frequency and amplitude; generating an error signal proportional to the phase difference between a signal representative of the output of the voltage-controlled oscillator and the measured response signal; adjusting the control signal to the voltage-controlled oscillator to cause the oscillatory force applied to the one mass to sweep within the calculated frequency range rendering the amplitude of the response signal to approach and exceed the calculated threshold values; and when the calculated threshold is exceeded by the amplitude of the response signal, finely adjusting the control signal to the voltage-controlled oscillator until the value of the measured error signal is equal substantially to the calculated final error value, whereupon the frequency of the response signal is the resonant frequency of the mechanical device.